Who were the Belgae ? Traditionally considered to be Gaulish Celts, they may in fact be a confederation of tribes of various ethnic origins. It is likely that the tribes in the east of modern Belgium were of Germanic origin, while those in northern France were actual Celtic Gauls. The Menapii were mentioned as Thracians.

Many have contended that the Belgae were a Gothic race ; but the only evidence adduced is a passage in Caesar, in which he says the ambassadors of the Rhemi informed him, that most of the Belgae were sprung from Germans who had passed the Rhine of old, and expelled the Gauls from those territories which they themselves then possessed. It does indeed appear from this passage that the Belgae came from beyond the Rhine ; but this is all that can be legitimately inferred from it. In the same place, Caesar clearly distinguishes the Condrusi, Eburones, Caerosi, and Paemani, from the rest of the Belgae, calling them the Germans who live on this side of the Rhine. These four tribes (who, he says, were called by the common name of Germans) were probably Gothic. But their language, and the distinction made by Caesar, clearly indicate that the rest were a different race.

Tribal composition of Belgium at the time of Julius Caesar

- Atrebates : in Artois and Hainaut. One group moved to England.

- Menapii : in the medieval County of Flanders (including French Flanders). The Luaighni tribe, who settled in County Meath, Leinster, Ireland, may be a branch of the Menapii.

- Nervii : in the medieval Duchy of Brabant (from the modern Walloon Brabant to North Brabant provinces) + part of the Hainaut province.

- Aduatuci / Tungri : in the Walloon section of the Meuse valley and the Hesbaye region.

- Eburones : in the modern provinces of Belgian and Dutch Limburg, the Hesbaye region, and North Rhineland between Aachen and Cologne. The site of their oppidum where Ambiorix sought refuge from Caesar's legions was probably in present-day Sinsin (near Marche-en-Famenne).

- Condrusi : in the Condroz and Fagnes region, on both sides of the Ourthe, south of the Meuse, and northern section of the Ardennes.

- Paemani : in the Famenne region, around the Lesse river.

- Treveri : around the modern province and Grand-Duchy of Luxembourg, in the Semois and Moselle region.

- Caeresi (or Caerosi) : near Luxembourg, possibly in the Eifel region around Prüm. Oppidum in Orchimont (Vresse-sur-Semois) near the Franco-Belgian border.

- Ubii : western Luxembourg

The Aduatuci

The Aduatuci (ot Atuatuci) were also refered to as Tungri (or Tongri). Their main cities were modern Namur (Oppidum Aduatucorum) and Tongeren (Aduatuca Tungorum).

Various sources mention that the Aduatuci were a branch of the Cimbri or of the Ambrones. The Cimbri originated from the Cimbric peninsula in Jutland, Denmark. Thought to be of Germanic descent, the Cimbri were clearly influence by the Celtic culture from the 3rd century BCE. In 113 BCE, they joined their neighbours the Ambrones* and the Teutoni*, and moved south to Gaul in search for empty lands, then split each in a different direction. The Cimbri had been travelling around Spain for two years, when in 103 BCE, they moved back to Gaul, and reunited with the Ambrones and Teutoni. They left warriors behind in the Meuse valley (Belgic Gaul), who became the Aduatuci.

* The Ambrones came from the region of the River Ambra (Emmer), a tributary of the Weser in northern Germany. The Teutoni's homeland was in between that of the Cimbri and Ambrones, in southern Jutland and northern Germany, between the Elbe and Oder rivers. They were described as Nordic people, with light skin, blond hair and blue eyes. Their language acquired a lot of Celtic words thanks to their active role in the amber trade between the Baltic and the Celtic homeland in the Alps.

The Eburones

Allies of the Aduatuci, the Eburones are said to descend from the Sicambri (or Sugambri), who had settled in the Rhine-Rhur region in the 1st century BCE. Their homeland is also associated with the south-eastern part of the modern Netherlands (Overijssel, Gelderland, Limburg). The Sicambri are also said to be the ancestors of the Franks, who resettled the Eburones territory from the 3rd century CE.

The Nervii

The Greek historian and geographer Strabo (64 BCE-24 CE) and the Roman senator and historian Tacitus (56–117) both describe the Nervians as a Germanic tribe. Tacitius had probably met the Nervians in person as his father was procurator of Belgica and Germania. In his book "Germania" (par. 28 and 29) Tacitus writes about the Nervii : "they say that this noble blood separates them completely from the Gauls, and from the Gaul laziness".

According to the Greco-Roman historian Appian, the Nervii were descended from the Cimbri, like the Aduatuci.

Their territory spread from the region of Cambrai in northern France to the Dutch province of North Brabant, encompassing most of the Belgian provinces of Hainaut, Walloon and Flelmish Brabant, and Antwerp. It could have spread as far as Nijmegen in Gelderland.